1. Field of the Invention
The present invention relates to a method and device for adjusting a focus bias of an objective lens for an optical disc on the basis of a level of a tracking error signal detected when the optical disc is driven.
2. Description of the Related Art
Conventionally, where the thickness of a substrate of an optical disc is thinner or thicker than the standard thickness, a high-frequency playback signal on a focus error zero point is weakened, a level of a tracking error signal is abruptly degraded, and the amount of jitter of the high-frequency playback signal is abruptly increased, when a general focus point, on the optical disc, of an objective lens (focus lens) arranged within an optical pickup, is not correct. Accordingly, it is necessary that the focus bias must be adjusted such that the focus point of the objective lens on the optical disc is optimally located.
A conventional focus bias adjustment method detects an error rate of playback data when the optical disc is reproduced, and adjusts the focus bias such that the error rate can be minimized. However, the conventional focus bias adjustment method is inefficient, because a single layer DVD (Digital Versatile Disc) needs about 3 seconds to adjust the focus bias, and a dual layer DVD needs about 6 seconds or more to adjust the focus bias.
Another conventional focus bias adjustment method using the jitter carries out a focusing-on operation, a tracking-on operation and a PLL (Phase-Locked Loop) lock operation, measures the amount of jitter from playback data, and adjusts the focus bias such that the amount of jitter can be minimized. But this conventional focus bias adjustment method using the jitter is also inefficient in that the amount of time for adjusting the focus bias increases.